In general, a sufficient length is required for an oxide superconducting wire employed in a practical superconducting apparatus. In order to manufacture a cable conductor having a capacity of at least 100 megawatts (MW) as a practical superconducting cable, for example, hundreds of oxide superconducting wires exhibiting a unit length of about 5 km as the final length of the superconducting cable are required. In this case, a wire (diameter: 0.9 mm, critical current: 20 A, temperature: 77 K) formed by bismuth oxide superconductor filaments coated with silver is employed as the oxide superconducting wire, for example.
As a superconducting magnet employed for a magnetic separator or a magnetic field generator, a magnet having an inner diameter exceeding 1 m is manufactured. In order to manufacture such a superconducting magnet, about 1000 oxide superconducting wires exhibiting a unit length of about 800 m per coil are required, for example. In this case, a tape-like wire (thickness: 0.25 mm, width: 4 mm, critical current: 50 A (temperature: 77 K)) formed by bismuth oxide superconductor filaments coated with silver is employed as the oxide superconducting wire.
At the current level of the technique of manufacturing an oxide superconducting wire, however, only a wire formed by bismuth oxide superconductor filaments coated with silver having a unit length of about several 100 m is manufactured. When the oxide superconducting wire of such a unit length has a single defective portion, the entire oxide superconducting wire of about several 100 m is regarded as defective, to disadvantageously result in a low manufacturing yield. Unless a technique of manufacturing an elongated oxide superconducting wire is developed, therefore, it is impossible at present to apply the current technique to the aforementioned practical superconducting apparatus. This is one of the primary factors for delay in the application of superconducting apparatus, which is an innovative technique, to industry and practical application thereof.
If a wire having a large unit length can be manufactured by connecting relatively short oxide superconducting wires with each other in order to implement the aforementioned superconducting cable having a capacity of at least 100 MW or a superconducting magnet employed for a magnetic field generator, it is possible to prepare a prototype apparatus for applying a superconducting apparatus to industry. Further, it is possible to understand the merits of the superconducting apparatus through the prepared prototype apparatus for progress in practical application.
However, the critical current of an oxide superconducting wire is disadvantageously reduced due to influence by strain resulting from deformation such as bending or tension. When end portions of oxide superconducting wires having a small unit length are superposed for connecting the oxide superconducting wires with each other by brazing or soldering, for example, the wires are bent through a guide roller or the like in the process of manufacturing a superconducting cable or a superconducting magnet and the critical current is reduced due to bending strain applied to the wires. This is because the junction formed by superposing the end portions with each other is hardly bent while the remaining portions are readily bent. Hence an end of the junction is bent with a bending radius smaller than the radius of the guide roller or the like when the end of the junction is bent through the guide roller or the like. As a result, a strain larger than an allowable bending strain for allowing the wires to maintain the critical current is applied to the end of the junction. Even if an oxide superconducting wire having a large unit length can be obtained by connecting the wires, therefore, the critical current is reduced due to influence by the strain applied to the end of the junction of the wire and hence it is disadvantageously difficult for a practical superconducting apparatus formed by the long wire to attain a prescribed function.
Accordingly, an object of the present invention is to provide a method of manufacturing an oxide superconducting wire which can manufacture the longest possible wire by connecting relatively short wires with each other and is capable of suppressing reduction of a critical current resulting from influence by strain also when the wire is bent after connection.
Another object of the present invention is to provide an oxide superconducting wire, a superconducting coil and a superconducting apparatus each comprising a connected portion, which can suppress reduction of an initial critical current of wires before connection also in a bent state.